Dichroic mirrors, that is mirrors that exhibit different colors in reflection and transmission, can be employed as rearview mirrors for motor vehicles and for other purposes. A mirror can be manufactured by applying a reflective coating to a glass substrate using magnetron sputtering techniques of the type described in Chapin, U.S. Pat. No. 4,166,018. Although a reflective metal such as chromium or silver may be employed as a reflective film for standard mirrors, dichroic mirrors commonly employ two contiguous films or films of materials having different refractive indices, and reflection occurs at the interface of these films.
Dichroic mirror assemblies that are used for rearview mirrors for vehicles may be provided with light sources such as light emitting diodes ("LEDs") carried in housings forward of the mirror so that light from the LEDs is transmitted rearwardly through the mirror. Examples of such mirror assemblies are found in Roberts, U.S. Pat. Nos. 5,481,409, 5,355,284 and 5,207,492, and Roberts et al. U.S. Pat. No 5,361,190.
Substantial optical performance is required of these dichroic mirrors, inasmuch as the mirrors must be highly reflective so that a motor vehicle operator can clearly see what is happening near the rear of the vehicle. Transmittance, of course, is important so that sufficient LED light passes through the mirror without the necessity of using extremely powerful and hence expensive LEDs. LEDs may signal a variety of events, both to the driver and to following traffic. LEDs may signal when doors have been opened, when brakes are applied, etc. Although some LED signals need only be bright enough to be readily perceived by a motor vehicle operator, others should be sufficiently bright so as to be perceived by vehicles following the vehicle. For example, it is difficult for following traffic to see when the side door of a passenger or cargo van has been opened. The appearance of a bright LED signal in the van's exterior rearview mirror can signal to following traffic that the door is open and passengers may be about to exit.
When curved mirrors are manufactured using magnetron sputtering processes, the glass substrates for the mirrors may first be bent to the desired shape, and then can be placed on an appropriate carrier and coated by magnetron sputtering. Due to curvature of the substrates, the reflective coatings that are produced may not be precisely uniform. The manufacturing process itself is tedious and time-consuming inasmuch as it requires multiple small glass substrates to be laid by hand upon a carrier that passes through a magnetron sputtering apparatus and requires each of the resulting individual mirror pieces to be removed by hand from the carrier sheet once the sputtering operation is completed.
It would be desirable to provide a dichroic mirror utilizing a flat glass sheet as the substrate, and thereafter bending and cutting the mirror as desired, with the mirror retaining good reflectance so that objects to the rear of the driver can be seen and good transmittance so LED signals can be easily perceived through the mirror. Unfortunately, the heat bending process, which may require temperatures ranging from about 650.degree. C. to about 800.degree. C., can produce undesired changes in commercially available dichroic mirrors. Reflectivity of such mirrors often is greatly decreased, and the development of haze may also present a problem. Haze in excess of 1%, in particular, should be avoided.